It is important to maintain the oil level of refrigeration compressors or like devices within a predetermined range in order to ensure efficient operation of the refrigeration system as well as preventing damage to the compressors. If the oil level in a compressor sump falls below a predetermined minimum level the compressor is automatically shut down to avoid or at least minimise damage created by the contact of unlubricated parts. Should the oil level in the compressor sump be above a predetermined maximum level, then large amounts of oil will be discharged with compressed refrigerant. This reduces the efficiency of the refrigeration system due to the insulating effect of the oil. This also effectively reduces the amount of oil available for any other compressors in the refrigeration system which may ultimately lead to the oil levels in other compressors falling below the predetermined minimum level shutting those compressors down.
It is essentially unavoidable that losses of oil will occur due to the inherent characteristics of refrigeration compressors. As oil is splashed about by the moving parts within the compressor a small percentage of the oil will be entrained in the compressed refrigerant. In an efficient system, there will typically be approximately 3% oil in circulation in the system. Oil losses may also occur due to leaks within the compressor and/or other devices within the refrigeration system through which compressor oil is held or refrigerant carrying oil passes.
Oil which is pumped out by the compressor with compressed refrigerant eventually returns normally as a large dose (slug) in a compressor suction line which feeds refrigerant vapour to the compressor for compression. When the refrigeration system includes a plurality of compressors arranged in parallel the volume of oil returned as a slug can be relatively large. Typically the oil slug returns after a defrost, or during periods of high evaporator load. The oil slug may not be evenly shared between the compressors, furthermore, any compressor that is not running will not receive any of the oil in the slug as there is no flow in the suction line of that compressor. The net effect is that the oil level in one or more of the compressors can exceed the predetermined maximum oil level while an oil reservoir of the refrigeration systems, designed to hold this oil is starved which in turn results in the starving of other compressors. It is not uncommon that the oil reservoir may be empty while some of the compressors are overfilled and others underfilled. The quick cure for this is to merely add more oil to the reservoir. This only serves to perpetuate the inefficiencies in the refrigeration system.
A system for controlling the oil level in the crank case or sump of a refrigeration compressor is described in U.S. Pat. No. 5,103,648 (AC & R Components, Inc) ACR!. The ACR system uses a plurality of optical sensors to determine the oil level within the sump. Each sensor has an output which is connected to the input of a control circuit which operates to energise a solenoid valve to add oil to the sump from a remote source when the oil level in the sump as sensed is below a predetermined level. As an optional feature, the system can also produce an alarm in response to the sump being either overfilled or underfilled with oil.
An admitted deficiency in the ACR system is that the optical sensors may be effected by the presence of foam in the oil as a result of dissolved refrigerant boiling out of the oil. To overcome this problem ACR propose use of timing circuits to cycle solenoid valves between their opened and closed conditions to dissipate the foam for more accurate sensing of the actual oil level. ACR also apparently fail to recognise the problems that may occur when compressors are over filled.